When a magnetic disk device is exposed to thermal fluctuations, such as a high temperature environment, data stored in the disk device may be corrupted or otherwise degraded. When the magnetic disk device reads such degraded data, signal quality of the read data may also be compromised. When degraded data is detected, the magnetic disk device executes a process of rewriting degraded data (such as a refresh process). However, when the magnetic disk device is reactivated after a sudden power loss occurs and while the magnetic disk device is exposed to thermal fluctuations, it is difficult for the magnetic disk device to detect data degraded by the thermal fluctuations. In the magnetic disk device, it is necessary to read all data in the disk and evaluate the read data in order to detect data degraded by thermal fluctuations. For that reason, when a large quantity of data are stored by the disk, the magnetic disk device performs a time-consuming process to detect data degraded by the thermal fluctuations.